Heartgard. Do you use it? Has it ever failed for you?

After hearing about heartworm positive dogs who were using heartgard, i looked into it a bit. Apparently, Heartgard has quite a failure rate. Although their package insert recommends fully chewing the tablet, they fail to mention that this is REQUIRED in order to get the full benefits of the preventative. Consequently, it seems some dogs have contracted heartworms while using heartgard. Has this happened to you or anyone you know?